a. Field of the Invention
This invention relates to vehicle brakes. In particular, the invention relates to a drum brake in which the brake actuating means engages the brake shoes at offset positions to improve the direction of the brake actuating forces.
b. Background Art
Referring to FIG. 1, a conventional prior art drum brake 10 is illustrated. In a conventional drum brake, a drum 12 rotates with a wheel or wheels proximate to one end of an axle. The drum 12 defines a radially inner braking surface 14. A brake spider 16 is disposed about the axle and a pair of brake shoes 18, 20 are pivotally mounted at one end to the brake spider 16. The opposite end of each brake shoe 18, 20 is engaged by an actuating member such as a cam 22 to move the brake shoes 18, 20 between positions of engagement and disengagement with the braking surface of the brake drum.
In a conventional S-cam drum brake as shown in FIG. 1, rollers 24, 26, or cam followers, are disposed between the brake shoes 18, 20 and the cam 22 to transfer actuating forces from the cam 22 to the brake shoes 18, 20. Force is applied by the cam 22 through the rollers 24, 26 along the direction indicated by arrows 28, 30, respectively, which extend from a “power circle” of the cam 22 defined by a radius from the rotational axis of the generally involute cam 22. The force vector represented by arrows 28, 30 may be resolved into two components—a component represented by arrows 32, 34 comprising the effective brake actuation force tangent to the pivot arc of the corresponding brake shoe 18, 20 and a component represented by arrows 36, 38 comprising the divergence between the direction of the force exerted by the cam 22 on rollers 24, 26 and the effective brake actuation force.
The divergence between the actuating forces applied by the cam 22 and represented by arrows 28, 30 and the effective brake actuating force represented by arrows 32, 34 has several drawbacks. First, the force component represented by arrows 36, 38 creates mechanical stress in the webs of the brakes shoes 18, 20 and is particularly acute when the force component 36, 38 is directed outward in trailing brake shoes. The increased stress can lead to cracks in the brake shoe webs and costly downtime and repairs. Second, the actuation efficiency of the brake 10 is less than optimal because the effective brake actuation force is less than the force exerted by the cam 22. As a result, more fluid pressure is required to actuate the brake leading to relatively large air chambers in brake actuators and/or requiring brake linings with relatively high friction coefficients. Third, the amount of rotation of cam 22 and, therefore, the displacement of brakes shoes 18, 20, is relatively limited thereby limiting the potential thickness of the brake linings and requiring more frequent maintenance and/or repair.
The inventor herein has recognized a need for a brake that will reduce one or more of the above-identified deficiencies and/or provide improved performance.